Questions/issues regarding generators

Andreas Rossberg rossberg at
Thu Mar 14 20:01:02 PDT 2013

On 15 March 2013 01:13, Brendan Eich <brendan at> wrote:
> Andreas Rossberg wrote:
>> See my reply to Jason: I think that in most practical cases (in
>> particular, all abstractions over iterables), the invocation of the
>> iterator method will happen inside an abstraction, and the programmer
>> does not have to worry about it.
> Talking 1:1 with you after the TC39 meeting, it came out that the ES6 spec
> does not say that iterators are iterables whose @iterator does "return
> self". That changes things, but still makes a messier contract than you
> prefer.

Yes. Iterators have a next method, that's all what makes them an
iterator, according to the wiki, and having an iterator method is
never mentioned there. The only place where such a case shows up in
the proposals is for generator objects.

> The contract you prefer is one where iterables have @iterator and calling it
> gets a (mutable) iterator that is *not* an iterable. It would require, as
> you proposed, making generators return iterables not iterators -- an extra
> allocation.
> At the level of contract cleanliness and usability, that may be better than
> the Pythonic convention -- I'm not sure. Cc'ing Jason.
> At the level of extra allocations, I still say "boo".

I'd say that one allocation per loop is perfectly affordable -- and is
likewise required for packaging up the return value. For both it is
easy to avoid ever materializing the extra object in the common case
of a for-loop.


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